The invention relates to chemical vapor deposition systems.
Chemical vapor processes for thin film fabrication pass a vapor over a substrate to either grow a film on the substrate, etch the substrate, or otherwise react with a material on the substrate to change the character of the substrate surface. For example, in a chemical vapor deposition (CVD) process for fabricating semiconductor devices, a flow of a reactive vapor is directed to an exposed side of a disk-shaped semiconductor wafer. The wafer is sometimes supported around the periphery of its bottom side on an annular-shaped ledge of an edge ring. The wafer's peripheral edge is left exposed. The substrate is sometimes rapidly heated to facilitate or speed the vapor processing, for example, in rapid thermal chemical vapor deposition (RTCVD) processes, rapid thermal oxidation processes (RTO), and rapid thermal nitridation (RTN) processes.
In such a system, the reactive gases can spill over the edge of the wafer and edge ring, depositing a non-uniform film on the peripheral edge of the wafer and on its backside. Non-uniform depositions on the edge or backside of the wafer can flake off and thereby generate particles that contaminate the process chamber. Also, non-uniform depositions are undesirable for subsequent wafer processing.
One approach to inhibit the process gases from depositing on the edge or backside is to use an edge ring that covers a portion of the upper surface of the wafer. Another approach is to coat the entire backside uniformly to produce a more stable film less likely to flake. To this end, the wafer is supported on pins so that the process gases can easily deposit on the backside. In those cases where depositing on the backside is undesirable, one or another of a variety of edge-specific purges with inert gases are used to prevent reactive gases from reaching the edge and backside areas. One type of such a system uses a susceptor with built-in channels for directing purge gas flows to the edge of the wafer.
Current schemes for providing effective edge purging may incompletely isolate the backside from reactive gases if the flow of purge gas is too weak. If the purge gas is flowed more strongly, it can spill over the front side of the wafer and mix with the process gas at the periphery of the wafer by diffusion or by convection. The resulting dilution of reactive gases over the front side of the wafer leads to incomplete film deposition near the periphery of the front side, thereby reducing the usable area having a uniform film on the wafer.